Low noise block supply and control voltage regulator

ABSTRACT

A low power circuit for providing power and control signals to a low noise block converter of a satellite receiver over a single coaxial cable includes a tracking switch-mode power supply. The power and control signals have a DC voltage level selected from a plurality of DC voltage levels and are modulated by an analog AC tone signal. The switch-mode power supply provides a regulated output voltage which tracks the selected DC voltage level. The regulated output voltage provides the input voltage to an adjustable linear amplifier which generates an output voltage having the selected DC voltage level modulated by the analog AC tone signal.

CROSS REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

BACKGROUND OF THE INVENTION

Satellite television receivers generally include a low noise blockconverter (LNB) at the satellite dish for controlling reception ofsatellite television signals. Functions of the LNB includedownconverting received satellite signals, changing the frequency bandof signal reception, changing the signal polarization of reception and,in some applications, controlling more than one receive antenna. Forthese purposes, the LNB requires control signals and power, both ofwhich are provided by circuitry housed in the box located near thetelevision set through which a user can change the channel of reception(sometimes referred to as the “set top box”). Since only a singlecoaxial cable couples the LNB to the set top box, the control and powersignals are carried by the single coaxial cable.

EUTELSAT, a European organization which governs television satellitecommunications, establishes specifications for the low noise blockconverter control and power signals. Analog AC tone control signals areprovided by a 22 kHz, 600 mV peak-to-peak signal which can be used toimplement DiSEqC™ (Digital Satellite Equipment Control) encoding for thepurpose of changing the polarization and frequency band of received RFsignals. Digital control information which is also used to change thefrequency band of received RF signals is transmitted as a nominal 13volt DC signal for a logic zero and as a nominal 18 volt DC signal for alogic one. In practice however, a logic zero corresponds more broadly tovoltages between approximately 12–14 volts and a logic one correspondsto voltages between approximately 17–20 volts. Power to the LNBcircuitry itself is provided by whatever DC voltage is being used totransmit digital control information at any given time. Further, the LNBcircuitry requires on the order of 0.6 amp of current.

Adjustable linear amplifiers are conventionally used to provide thecontrol and power signals to the LNB. In this arrangement, the linearamplifier provides an AC modulated output voltage which has anadjustable DC voltage level in order to provide a logic one voltagelevel or a logic zero voltage level in response to control signals froma microprocessor within the set top box. In order to provide the logicone voltage levels, the minimum input voltage to the linear amplifier isrequired to be on the order 20 volts. However, when a logic zero voltagelevel of 12 volts is selected, the resulting power dissipation of thelinear amplifier approaches 5 watts.

One attempt at lowering the power dissipation associated with the use ofa linear amplifier to provide power and control signals to an LNB is theLNBP10 Series LNB Supply and Control Voltage Regulator product of STMicroelectronics of Lincoln, Mass. In this product, two input voltagelevels are available to the linear amplifier, depending on which outputvoltage level is selected. The input voltage levels are provided by anunregulated AC/DC converter.

While the ST product somewhat reduces power dissipation as compared tothe conventional linear amplifier arrangement, use of the ST productrequires two windings on the power transformer of the AC/DC converter,only one of which is in use at any given time, thereby adverselyimpacting the efficiency of the converter. Further, even with two inputvoltage levels, the voltages dropped across the linear amplifier arestill significant. As a result, power dissipation is still high enoughto require relatively large package dimensions due to power dissipationand thermal considerations.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a low power circuit forgenerating power and control signals for a low noise block converter(LNB).

It is a further object of the invention to provide an LNB supply andcontrol voltage regulator which can be housed in a smaller package withreduced heat sinking requirements than heretofore possible.

These and other objects of the invention are provided by an LNB supplyand control voltage regulator which includes a DC/DC switch-mode powersupply providing a regulated output voltage to a linear amplifier. Theoutput voltage of the linear amplifier has an adjustable DC voltagelevel and is modulated by an analog AC tone signal to provide power andcontrol signals to an LNB of a satellite receiver over a singleconductor. The regulated output voltage of the DC/DC switch-mode powersupply tracks the selected DC voltage level, so as to minimize thevoltage drop across the linear amplifier.

More particularly, the switch-mode power supply has an input port towhich a DC input voltage is applied, a feedback port responsive to areference voltage indicative of the selected DC voltage level, and anoutput port at which the regulated, tracking output voltage is provided.The regulated output voltage is greater than the selected DC voltagelevel by a predetermined offset voltage. The linear amplifier has aninput port coupled to the output port of the switch-mode power supply, acontrol port to which the reference voltage indicative of the selectedDC voltage level is applied, and an output port at which an outputvoltage having the selected DC voltage level modulated by the analog ACtone signal is provided.

With this arrangement, the voltage dropped across the linear amplifieris minimized since the input voltage to the linear amplifier ismaintained at the predetermined offset voltage greater than theamplifier's output voltage. The resulting lower power dissipationpermits the LNB supply and control voltage regulator to be provided inthe form of a monolithic integrated circuit having relatively smallpackage dimensions, since heat sinking requirements are reduced.

It will be appreciated by those of ordinary skill in the art that theswitch-mode power supply is capable of generating the different DCvoltages necessary to provide the digital control signals and power tothe LNB. However, use of the linear amplifier is desirable to inject theanalog AC tone signal. Thus, while for most applications, the use of aswitch-mode power supply to provide the input voltage to a linearamplifier would be considered redundant, in the described satellitereceiver application, this combination provides significant benefits.Specifically, the linear amplifier facilitates modulation by the analogAC tone signal and the switch-mode power supply provides a trackinginput voltage to the linear amplifier in order to significantly reducethe power dissipation otherwise associated with the linear amplifier.

The LNB supply and control voltage regulator further includes a signalgenerator for generating the analog AC tone signal and for applying theanalog AC tone signal to the linear amplifier to modulate the outputvoltage of the linear amplifier. In one embodiment, the analog AC tonesignal modulates the reference voltage input to the linear amplifier andin another embodiment, the analog AC tone signal modulates the feedbackinput of the linear amplifier.

In applications in which the LNB supply and control voltage regulatorcontrols more than one receive antenna, the output port of the linearamplifier includes first and second output port portions and the outputvoltage is provided at a selected one of the first and second outputport portions in response to an output port control signal.

In one embodiment, the switch-mode power supply has a buck topology andincludes a current mode pulse-width-modulation controller. However, itwill be appreciated that other converter topologies and controltechniques may be used while still realizing the advantages of thepresent invention.

Also described is a method for providing power and control signalsselected from a plurality of DC voltage levels and being modulated by ananalog AC tone signal to satellite receiver apparatus on a singleconductor. The method includes selecting one of the plurality of DCvoltage levels and providing a regulated output voltage with aswitch-mode power supply, with the regulated output voltage tracking theselected DC voltage level. The method further includes applying theregulated output voltage to a linear amplifier which provides an outputvoltage having the selected DC voltage level which is modulated by theanalog AC tone signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of this invention, as well as the inventionitself, may be more fully understood from the following description ofthe drawings in which:

FIG. 1 is a block diagram of a portion of a satellite televisionreceiver system including an LNB supply and control voltage regulatoraccording to the invention;

FIG. 2 is a block diagram of the LNB supply and control voltageregulator of FIG. 1;

FIG. 3 is a simplified schematic of an illustrative switch-mode powersupply for use in the LNB supply and control voltage regulator of FIG.2; and

FIGS. 4 and 4A are a schematic of the LNB supply and control voltageregulator of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a portion 10 of a satellite television receiversystem includes a satellite dish 14 having a low noise block converter(LNB) 34 for controlling reception of satellite television signals. TheLNB 34 receives power and control signals from a set top box 16 via asingle coaxial cable 18. The set top box 16 typically sits on top of, oradjacent to a television 20 and is response to input signals from theviewer to vary the frequency of signal reception. To this end, the settop box 16 includes a microprocessor 24 and an LNB supply and controlvoltage regulator 30 according to the invention.

The set top box 16 receives a DC input voltage, Vin, from an offlineAC/DC converter 26 which is further coupled to a standard 120–240 voltAC source 28, as shown. The offline AC/DC converter 26 provides the Vinvoltage between 22–47 volts.

As noted above, the control signals to the LNB 34 include an analog ACtone signal in the form of a 22 kHz, 600 mV peak-to-peak signal anddigital control signals in the form of a nominal 13 volt DC signal for alogic zero and a nominal 18 volt DC signal for a logic one. In practicehowever, a logic zero corresponds more broadly to voltages betweenapproximately 12–14 volts and a logic one corresponds to voltagesbetween approximately 17–20 volts. Power to the LNB circuitry itself isprovided by the digital control signal voltage. The LNB circuitryrequires a current on the order of 0.6 amp.

The LNB supply and control voltage regulator 30 provides power andcontrol signals to the LNB 34 in the form of an output signal, LNB,which has a DC voltage level selected from a plurality of levels toprovide a logic one or logic zero and which is modulated by the analogAC tone signal. The LNB signal is provided to the LNB 34 at thesatellite dish 14 via coaxial cable 18. The LNB supply and controlvoltage regulator 30 is responsive to signals 38, 44 for selecting theappropriate DC voltage level for the LNB signal. More particularly, themicroprocessor 24 generates control signals 38 in response to user inputsignals 44 and the LNB supply and control voltage regulator 30 isfurther responsive to one or more additional user inputs 44, as will bedescribed. It will be appreciated by those of ordinary skill in the artthat the user input signals 44 may be provided by various user interfaceapparatus, such as with a remote control or with front panel switches.

Referring also to FIG. 2, the LNB supply and control voltage regulator30 includes a switch-mode power supply 50 having an input port 52 a towhich the Vin voltage is applied, a feedback port 52 b to which areference voltage, Vref, is applied, and an output port 52 c at which aregulated output voltage, Vbulk, is provided on signal line 94. The Vrefvoltage 62 is generated by a reference voltage generator 60 and isindicative of the selected DC voltage level for the LNB signal on line18. The Vbulk voltage tracks the selected DC voltage level for the LNBsignal. In particular, the Vbulk voltage is maintained at apredetermined amount greater than the voltage level of the LNB signal.

The LNB supply and control voltage regulator 30 further includes alinear amplifier 56 having an input port 58 a coupled to the output port52 c of said switch-mode power supply 50, a control port 58 b to whichthe Vref voltage is applied for selecting the DC voltage level of theLNB signal, and an output port 58 c at which the LNB signal having theselected DC voltage level and being modulated by an analog AC tonesignal is provided.

The Vref voltage 62 sets, and is indicative of the DC voltage level ofthe LNB output signal. Thus, the Vref voltage is preferably provided bya high precision voltage source in order to ensure that the LNB signalrepresents the correct digital control information. Various circuits aresuitable for generating a precise Vref voltage. One suitable referencevoltage generator 60 is shown in FIG. 4 and described below.

In the illustrative embodiment, the Vref voltage 62 is equal toone-sixth of the selected DC voltage level of the LNB signal. It will beappreciated by those of ordinary skill in the art however that the Vrefvoltage may alternatively be equal to or may be otherwise proportionalto, or indicative of the selected DC voltage level.

The Vref voltage 62 is generated by the reference voltage generator 60in response to a VSEL0 control signal and a VSEL1 control signal fromthe microprocessor 24 (collectively labeled 38 in FIG. 1) and to an LLCuser input signal 44. The VSEL0 and VSEL1 control signals in turn aregenerated by the microprocessor 24 in response to user input signals 44,such as a user channel selection selected via a remote control. The LLCuser input signal 44 is provided by a switch setting on the set top box16.

In the illustrative embodiment, the DC voltage level of the LNB signalis selected from 12, 13, 18 and 20 volts. Further, the LNB signalvoltage optionally may be increased by 1 volt in order to compensate forvoltage drop in the coaxial cable 18 (FIG. 1). A logic high LLC signalserves to increase the DC voltage of the LNB signal by 1 volt.

In the illustrative embodiment, the DC voltage level of the LNB signalis selected according to the following table and the reference voltage62 is generated at a voltage level equal to one-sixth of the selected DClevel:

VSEL0 VSEL1 LLC = 0 LLC = 1 0 0 13 volts 14 volts 0 1 18 volts 19 volts1 0 12 volts 13 volts 1 1 20 volts 21 volts

The analog AC tone signal 64 is generated by an AC signal source 68 andcan be applied the linear amplifier 56 to modulate its output voltage byvarious techniques. The block diagram of FIG. 2 illustrates one suchtechnique in which the analog AC tone signal 64 modulates the Vrefvoltage 62 under the control of a switch SW1. Switch SW1 is controlledby an ENT tone enable signal 38 generated by the microprocessor 24 inresponse to a user input 44. More particularly, the microprocessor 24uses a look up table to determine which of two possible antennapolarizations is required to receive RF signals corresponding to aparticular channel selected by a user input 44. An alternative techniquefor applying the analog AC tone signal to the linear amplifier 56 isillustrated in FIG. 4 and described below.

The Vbulk output voltage 94 of the power supply 50 is maintained at avoltage level that is a predetermined amount greater than the LNB outputsignal. Further, the predetermined amount, or offset between the LNBsignal and the Vbulk voltage is selected to be as small as possiblewhile still providing sufficient input voltage to enable the linearamplifier to supply the selected output voltage, given the forwardresistance and current supplied by the linear amplifier. In theillustrative embodiment, the predetermined offset voltage is on theorder of 0.8 volts.

With this arrangement, the voltage drop across the linear amplifier 56is maintained at the relatively low offset voltage and the powerdissipation associated with the linear amplifier 56 is likewiseminimized. In the illustrative embodiment, the power dissipationassociated with the linear amplifier is on the order of one-half watt.Such lower power dissipation advantageously permits the monolithic LNBsupply and control voltage regulator 30 to be provided in a relativelysmall and inexpensive package with lesser heat sinking requirements thanconventionally possible. Thus, while in most applications, the use of aswitch-mode power supply to provide the input voltage to a linearamplifier would be considered redundant, in the described satellitereceiver application, this combination provides significant benefits.

Referring also to FIG. 3, an illustrative buck topology for theswitch-mode power supply 50 of the LNB supply and control voltageregulator 30 is shown. It will be appreciated by those of ordinary skillin the art that other power supply topologies, such as a boostconverter, may be utilized while still realizing the advantages of thepresent invention. The illustrated buck converter 50 is preferred sinceit steps down the Vin voltage with relatively high efficiency and lowlosses.

Input port 52 a of the power supply 50 receives the DC Vin voltage andis coupled to a first terminal 82 a of a power transistor 80. A secondterminal 82 b of the transistor 80 is coupled to an inductor 84 and acontrol terminal 82 c of the transistor 80 is responsive to a transistordrive signal 110 from a pulse-width-modulation (PWM) controller 112. Theillustrated transistor 80 is a N-type Metal Oxide Field EffectTransistor (MOSFET). However, it will be appreciated by those ofordinary skill in the art that other transistor types may be suitablefor use in the power supply 50. In the illustrative embodiment, the Vinvoltage is on the order of 22 to 47 volts and Vbulk output voltage online 94 is between 12–21 volts.

The inductor 84 is further coupled to a capacitor 90 and to the outputterminal 52 c of the power supply at which the regulated Vbulk voltageis provided. A free-wheeling diode 92 is coupled across theseries-coupled inductor 84 and capacitor 90, as shown.

The PWM controller 112 includes an offset voltage generator 102, anerror amplifier 98, and a PWM comparator 108, as shown. The Vbulk outputvoltage is divided by resistor divider 132 and fed back to the invertinginput terminal of the error amplifier 98. In the illustrativeembodiment, the resistor divider 132 divides the Vbulk voltage by afactor of six. The Vref voltage is coupled to the non-inverting inputterminal of the error amplifier 98 through the offset voltage generator102, as shown. The offset voltage generator 102 adds a predeterminedoffset voltage, Voffset, to the Vref voltage.

The Ve error voltage 104 is coupled to the PWM comparator 108 whichgenerates the transistor drive signal 110 by comparison of the Ve errorvoltage to a sawtooth waveform. In the illustrative embodiment, the PWMcontroller 112 is a current mode controller and thus, the sawtoothwaveform is indicative of the power supply output current waveform.However, it will be appreciated by those of ordinary skill in the artthat voltage mode controllers are likewise suitable for use in the powersupply 50. The transistor drive signal 110 varies the duty cycle (ratioof on time to off time) of the transistor 80 as necessary in order tomaintain the Vbulk voltage at a voltage level equal to the Voffsetvoltage plus the selected DC voltage level for the LNB signal. The Vbulkoutput voltage is equal to the Vin input voltage multiplied by the dutycycle of the transistor 80.

In operation, when the transistor 80 is on, the diode 92 isreverse-biased and the output current is supplied from the input sourceVin through the transistor 80. When the transistor 80 is off, the diode92 conducts and the output current is supplied by the inductor 84 andcapacitor 90. The inductor 84 and capacitor 90 form an LC filter forreducing the ripple component of the Vbulk output voltage.

In the illustrative embodiment, the switching frequency of the powersupply 50 is on the order of 352 KHz. However, it will be appreciated bythose of ordinary skill in the art that the switching frequency can bereadily varied in order to optimize known tradeoffs, such as componentcost and the size of the LC filter.

With the above-described power supply 50, the Vbulk output voltage ismaintained at a voltage level that is greater than the selected DCvoltage level for the LNB signal by an amount equal to the Voffsetvoltage. Thus, only the Voffset voltage is dropped across the linearamplifier 56. In this way, the power dissipation associated with thelinear amplifier 56 is minimized.

Referring also to FIGS. 4 and 4A, a schematic of an LNB supply andcontrol voltage regulator 30′ according to the invention is shown inwhich like components are designated by like characters. The regulator30′ is substantially similar to the regulator 30 of FIG. 2 with thefollowing exceptions. First, the regulator 30′ of FIG. 4 applies theanalog AC tone signal to modulate the linear amplifier output signal bya different technique than is shown in FIG. 2. Further, the regulator30′ of FIG. 4 is designed to control a selected one of two receiveantenna, or antenna heads. For this purpose, the regulator 30′ includesa first output port LNBA and a second output port LNBB and the LNBsignal is provided at a selected one of the LNBA and LNBB ports, as willbe described. The regulator 30′ includes additional circuitry not shownin the regulator 30 of FIG. 2, such as current limit and thermalshutdown circuitry, as will be described.

In the illustrative embodiment, the components within the border 100 areincorporated into a monolithic integrated circuit (IC). It will beappreciated by those of ordinary skill in the art however, that theregulator 30′ may be provided entirely by discrete components or thecomponents that are incorporated into the IC may be varied (i.e., somecomponents illustrated as being on the IC may be provided off-chip andsome components that are illustrated as being provided off-chip may beincorporated into the IC). In the illustrative embodiment, the IC isprovided in a 24 pin dual in-line package (DIP) or an Small Outline IC(SOIC) power-tab package.

The LNB supply and control voltage regulator 30′ includes switch-modepower supply 50, linear amplifier 56, reference voltage generator 60,and analog AC tone signal generator 68. The reference voltage generator60 includes a bandgap voltage generator 120 which provides a pluralityof precise voltage outputs 122. A voltage selector 124 is coupled to theoutputs 122 of the bandgap voltage generator 120 and is responsive toVSEL0, VSEL1 control signals 38 from the microprocessor 24 (FIG. 1) andthe LLC user input signal 44 for selecting the DC voltage level of theLNB signal and the Vref voltage 62 which, in the illustrativeembodiment, is one-sixth the LNB signal voltage. The Vref voltage 62 iscoupled to the linear amplifier 56, as shown. An external capacitor maybe coupled to a TCAP terminal 66 of the IC 100 in order to set the riseand fall times of the LNB output signal by adjusting the slew rate ofthe Vref voltage 62.

The linear amplifier 56 includes an error amplifier 130 having anon-inverting input coupled to the Vref voltage 62, a first invertinginput which receives a feedback signal from the output of the erroramplifier through resistor divider 132, and a second inverting input 134which receives a current limit signal. In operation, the error amplifier130 maintains its output voltage at the level of the Vref voltage unlessan overcurrent condition exists, in which case the output voltage of theerror amplifier is decreased in order to thereby decrease the outputcurrent provided by the linear amplifier.

The linear amplifier 56 further includes a buffer 138 which has a firstinput coupled to the output of error amplifier 130 and a second input towhich the AC tone signal 64 is applied. The buffer 138 modulates theoutput of the error amplifier 130 by the AC tone control signal. Thus,in the embodiment of FIGS. 4 and 4A, the AC tone signal 64 is applied tothe feedback node of the linear amplifier 56; whereas, in FIG. 2, the ACtone signal 64 is applied to the Vref voltage.

A logic input signal OSEL provided to the IC 100 is used to selectbetween the two output ports LNBA and LNBB of the linear amplifier 56.In particular, the OSEL signal controls a pair of switches 142, 144. Inthe illustrative embodiment, when the OSEL signal is at a logic highlevel, the LNBA output is disabled and the LNBB output is enabled;whereas, when the OSEL signal is at a logic low level, the LNBA outputis enabled and the LNBB output is disabled. Each of the LNBA and LNBBoutputs is coupled to a respective coaxial cable 18 a, 18 b which isfurther coupled to a respective antenna, or antenna head at a satellitedish 14 (FIG. 1).

More particularly, when the OSEL signal is at a logic high level, thefirst switch 142 couples the output of the linear amplifier buffer 138to an LNBA output stage 156 and also couples the feedback input of thebuffer 138 to the LNBA output stage 156. When the OSEL signal is at alogic low level, the second switch 144 couples the output of the linearamplifier buffer 138 to an LNBB output stage 158 and also couples thefeedback input of the buffer 138 to the LNBB output stage 158. Outputstages 156, 158 are capable of sourcing the required 0.6 amps of currentto the LNB 34.

It will be appreciated by those of ordinary skill in the art that thetopology of the linear amplifier 56 may be varied while still providingthe functionality of buffering the Vref voltage 62 and modulating thelinear amplifier output signal by the analog AC tone signal 64.

The switch-mode power supply 50 includes the power MOSFET 80 having afirst terminal 82 a to which the Vin voltage is coupled, a secondterminal 82 b coupled to the inductor 84 and to diode 92 and a controlterminal 82 c responsive to transistor drive signal 110. The transistordrive signal 110 is provided by the current mode PWM controller 112. Theinductor 84 is further coupled to capacitor 90 across which theregulated Vbulk voltage is provided on signal line 94, as shown.

The Vbulk voltage is fed back to the PWM controller 112, and inparticular is coupled to the inverting input terminal of the erroramplifier 98. The non-inverting input terminal of the error amplifier 98is coupled to the output of the linear amplifier error amplifier 130through resistor divider 132 and offset voltage generator 102. Theresistor divider 132 divides the LNB output voltage by six in theillustrative embodiment. The current sense PWM comparator 108 compares awaveform indicative of the Vbulk current to a voltage equal to theVoffset voltage plus the Vref voltage in order to provide the transistordrive signal 110 with the necessary duty cycle to maintain the Vbulkvoltage at the LNB voltage plus the offset voltage. In the illustrativeembodiment, the PWM controller 112 incorporates slope compensation.

An internal regulated supply 146 receives the Vin voltage and provides aregulated output voltage on signal line 148 to the IC circuitryincluding a charge pump 152. The charge pump 152 generates a voltage,VPUMP, which is on the order of 6V greater than the Vin voltage forpowering the IC circuitry. The charge pump 152 further provides adisable signal 154 to the PWM controller 112 in order to disable the PWMcontroller if the VPUMP voltage falls below a predetermined level belowwhich prevents the FET from conducting fully.

The analog AC tone signal generator 68 includes an n×22 KHz clockgenerator 160 providing a clock signal to the clock input of the PWMcontroller 112 and to a divider 164. In the illustrative embodiment, theclock generator 160 generates a clock signal 162 having a frequency of352 KHz factory trimmed to +/−2 KHz. The divider 164 divides thefrequency of the clock signal 162 by n, such as n=16 in the illustrativeembodiment, to provide the 22 KHz AC tone signal 64 to a slew controlcircuit 168. The output of the slew control circuit 168 is coupled tothe switch SW1 which is controlled by the ENT tone enable controlsignal. The ENT signal activates modulation of the linear amplifieroutput signal with the analog AC tone signal of 0.6 volts peak-to-peakand 22 KHz+/−2 KHz.

The analog AC tone signal generator 68 further includes an externalmodulation interface 170 which permits the linear amplifier outputsignal to be modulated by an external signal applied to an EXTM terminalof the IC 100.

The regulator 30′ further includes current limit and thermal shutdowncircuitry. The current limit circuit includes a comparator 180 andprovides short circuit protection to the LNBA and LNBB outputs. Thecomparator 180 has an inverting input terminal responsive to the Vbulkvoltage and a non-inverting input terminal responsive to the Vbulkcurrent as provided by sense resistor 182 through an offset voltagegenerator 184. When the sensed current exceeds the Vbulk output voltageby an amount equal to the offset voltage 184, the output of the currentlimit comparator 180 transitions to a logic high level and causes thelinear amplifier error amplifier 130 to decrease its output voltage andthus also to decrease the DC level of the LNB output voltage. Themaximum current is set by adjusting the value of the sense resistor 182.In particular, in the illustrative embodiment, the maximum current isequal to 0.15 divided by the value of the sense resistor 182.

Various circuitry and techniques are suitable for monitoring for anovertemperature condition in order to provide a thermal shutdownfunction. As one example, a fixed voltage may be applied to a Darlingtontransistor 176 and the Vbe voltage monitored. An overtemperaturecondition is indicated if the Vbe voltage falls to below a predeterminedlevel.

An OLF pin 190 of the IC 100 permits diagnostic information to beobtained. In particular, an over current condition, a short circuitcondition, or thermal shutdown condition causes the voltage at the OLFterminal to transition to a logic low level in order to thereby indicatea fault condition.

The regulator 30′ is capable of operating in a standby mode under thecontrol of a logic input signal, EN. When the EN signal is at a logiclow level, then both the LNBA and LNBB output signals are disabled. Thisfeature allows the antenna downconverters to be supplied and controlledby other satellite receivers sharing the same coaxial cable 18 (FIG. 1).

Having described the preferred embodiments of the invention, it will nowbecome apparent to one of ordinary skill in the art that otherembodiments incorporating their concepts may be used.

It is felt therefore that these embodiments should not be limited todisclosed embodiments but rather should be limited only by the spiritand scope of the appended claims. For example, it will be appreciated bythose of ordinary skill in the art that the circuitry and techniquesdescribed herein provide the same advantages in satellite systems otherthan television satellite systems. All publications and references citedherein are expressly incorporated herein by reference in their entirety.

1. A circuit for providing a power and control signal selected from aplurality of DC voltage levels and being modulated by an analog AC tonesignal to satellite receiver apparatus on a single conductor,comprising: a switch-mode power supply having an input port to which aninput voltage is applied, a feedback port responsive to a referencevoltage indicative of said selected DC voltage level, and an output portat which a regulated output voltage is provided, wherein said regulatedoutput voltage is greater than said selected DC voltage level by apredetermined amount; and a linear amplifier having an input portcoupled to said output port of said switch-mode power supply, a controlport to which said reference voltage indicative of said selected DCvoltage level is applied, and an output port at which an output voltagehaving the selected DC voltage level and being modulated by the analogAC tone signal is provided.
 2. The circuit of claim 1 wherein saidsatellite receiver apparatus comprises a low noise block converter of asatellite television system.
 3. The circuit of claim 1 furthercomprising a signal generator for generating said analog AC tone signaland for applying said analog AC tone signal to said linear amplifier. 4.The circuit of claim 1 wherein said switch-mode power supply is a buckconverter.
 5. The circuit of claim 1 wherein said switch-mode powersupply is a boost converter.
 6. The circuit of claim 1 wherein saidoutput port of said linear amplifier comprises a first output portportion and a second output port portion and wherein said output voltageof said linear amplifier is provided at a selected one of said first andsecond output port portions in response to an output port controlsignal.
 7. A method for providing a power and control signal selectedfrom a plurality of DC voltage levels and being modulated by an analogAC tone signal to satellite receiver apparatus on a single conductor,comprising the steps of: selecting one of said plurality of DC voltagelevels; providing a regulated output voltage with a switch-mode powersupply, said regulated output voltage having a voltage level greaterthan said selected DC voltage level by a predetermined amount; andapplying said regulated output voltage to a linear amplifier, saidlinear amplifier providing an output voltage having said selected DCvoltage level and being modulated by said analog AC tone signal.
 8. Themethod of claim 7 further comprising the step of providing said outputvoltage of said linear amplifier to a low noise block converter of asatellite television system.
 9. The method of claim 7 further comprisingthe steps of: generating said analog AC tone signal; and applying saidanalog AC tone signal to said linear amplifier.
 10. The method of claim7 wherein said linear amplifier provides said output voltage at aselected one of a plurality of output ports.
 11. A circuit for providinga power and control signal selected from a plurality of DC voltagelevels and being modulated by an analog AC tone signal to a low noiseblock converter of a satellite television system on a single coaxialcable, comprising: a switch-mode power supply having an input port towhich an input voltage is applied, a feedback port to which a referencevoltage indicative of said reference voltage level is applied, and anoutput port at which a regulated output voltage is provided, whereinsaid regulated output voltage is greater than said selected DC voltagelevel by a predetermined amount; a linear amplifier having an input portcoupled to said output port of said switch-mode power supply, a controlport to which said reference voltage indicative of said selected DCvoltage level is applied, and an output port at which an output voltagehaving the selected DC voltage level is provided; and a signal generatorfor generating said analog AC tone signal and for applying said analogAC tone signal to said linear amplifier, wherein said output voltage ofsaid linear amplifier is modulated by said analog AC tone signal. 12.The circuit of claim 11 wherein said switch-mode power supply comprises:an error amplifier having a first input responsive to said referencevoltage, a second, feedback input, and an output at which an errorsignal is provided; a pulse-width-modulation comparator responsive tosaid error signal for providing a transistor drive signal; a transistorhaving a first terminal to which said input voltage is applied, a secondterminal, and a control terminal responsive to said transistor drivesignal; and an inductor having a first terminal coupled to said secondterminal of said transistor and a second terminal at which said outputvoltage of said linear amplifier is provided, wherein said outputvoltage is coupled to said feedback input of said error amplifier. 13.The circuit of claim 12 further comprising an offset voltage generatorcoupled between said reference voltage and said first input of saiderror amplifier.
 14. The circuit of claim 12 wherein said erroramplifier and said pulse-width-modulation comparator comprise a currentmode pulse-width-modulation controller.